I have been using a cabin near Big Sur, California that was built in the 60s. The cabin has a natural spring-fed tank as a water source, and the building is supplied by old 3/4" galvanized pipe running underground for a length of ~60 ft. The 3/4" pipe has become partially exposed and rusty and recently sprung a leak where it crosses the "driveway" into the cabin property. The water into the cabin also contains a lot of rust.

I would like to replace the galv. pipe entirely with an underground run of PEX. I was planning to run the PEX through larger diameter PVC pipe to protect it, prevent potential damage from cars driving over the area, and make it easier to replace in the future. However, I'm not sure how to properly connect the PEX to the large diameter steel fittings at either end. I estimate that the pipe coming down from the hill is 3-5" OD, but I haven't dug up its junction with the 3/4" pipe yet. The large pipe also feeds a water tap for firefighters, so I don't want to mess with that.

If I connect to steel underground at either end using brass fittings, won't they corrode rapidly, especially when the ground gets wet with rain?

Should I run the PVC conduit protecting the PEX up out of the ground to keep the steel-brass connections dry?

What type of PEX is recommended? Do I need oxy barrier PEX or will A or B be fine in this application?

This area gets really hot during the summer, but rarely drops below freezing. The pipe run is underground in an area that gets full sun most of the day.

 

PEX sounds like a reasonable solution. Most water services are done in PE (black) pipe since it's cheaper than PEX. But both are rated for underground use.

Brass fittings won't rust, so no issue using them underground. Or you can use stainless barb fittings with PE pipe.

I do like the idea of protecting the pipe with PVC, it also makes it easier to pull a new pipe if/when you need to replace it in 50 years. I believe you'll need to get it at least 12" deep to be protected from cars driving over it. (dig the trench 14-16" deep so the top of the pipe is more than 12" deep). The PVC won't really help much with protecting from compression - but at that depth, you should be fine.

 

PEX is great but most it is NOT resistant to ultra violet light. You will need to completely protect it from sunlight. You can use PVC or black poly pipe as your conduit for the PEX.